The conventional power transmission which transmits power of an engine for driving one of the front wheel and rear wheel to the other by operating a hydraulic clutch, has been well-known as disclosed in the Japanese Patent Laid-Open Gazette No. Sho 60-252,026.
The above power transmission interposes between a first rotary shaft for transmitting the driving power to the front wheels and a second one for transmitting the same to the rear wheels an oil pump driven by a rotational speed difference between both the rotary shafts and clutch means operated by oil pressure of the oil pump, so that, when a rotational speed difference is created between the first rotary shaft and the second one, the oil pump issues discharge pressure corresponding to the rotational speed difference, thereby connecting the clutch means through the discharge pressure to automatically transmit a torque to the second rotary shaft.
Such conventional power transmission, which is interposed between the first and second rotary shafts, is complicated in construction and its mounting position is restricted, thereby creating the problem in that there is no degree of freedom as to the mounting position. Also, a pressurized chamber or room for a piston which actuates the clutch means communicates with the suction side of the oil pump through orifices through which internal pressure at the pressurized room is controlled, but communicates through an open circuit open to the atmosphere, so that a separate oil tank is needed and the power transmission is large-sized as a whole and also a particular oil passage is required between the oil pump and the oil tank, thereby creating the problem in that the power transmission is complicated in construction and expensive to produce.